1. Field of the Invention
The present invention relates to a dual-power-supply system using both a fuel cell that generates power by a chemical reaction of fuel such as methanol or hydrogen, and a chargeable secondary battery such as a lithium ion battery.
2. Description of the Related Art
Portable information apparatuses such as cellular phones, PDAs, and digital cameras mainly use chargeable secondary batteries such as lithium ion batteries as power supplies. Recently, as demands have arisen for high-performance, high-speed, multifunctional apparatuses that can be driven for long time periods, as the power supplies, it is anticipated that compact fuel cells will be used, and some fuel cells are beginning to be manufactured or used in trials.
Fuel cells do not require to be charged, unlike conventional secondary batteries, and can drive apparatuses for long time periods by merely replenishing the fuel or exchanging fuel cartridges. Of these fuel cells, a fuel cell using methanol or the like as the fuel can provide a stable output for a long time but cannot provide output corresponding to the load condition of an apparatus to be used, owing to the characteristics of fuel cells. In particular, an apparatus such as a cellular phone or digital camera has a plurality of operation modes, and the necessary output of one mode may be twice or more that of another mode. Accordingly, a fuel cell cannot singly be used in any apparatus in such manner.
When using a fuel cell in an electronic apparatus as described above, therefore, a general approach is to use, e.g., a chargeable lithium ion secondary battery capable of coping with load fluctuations, in addition to the fuel cell. For example, U.S. Pat. No. 6,656,618 has disclosed a system using both a fuel cell and secondary battery by converting the output voltage from the fuel cell by a DC-DC converter.